The present invention relates to an apparatus for measuring, using a laser beam, the attitude angles of a motor vehicle, in particular the toe-in and camber angles of the associated wheels.
It is known that the attitude angles of a motor vehicle, in particular, as mentioned, the toe-in and camber angles of the wheels, determine the behaviour of the vehicle on the road, both as regards the so-called xe2x80x9croad-holdingxe2x80x9d performance and the travel comfort. The importance of carrying out frequently an accurate check as to the correctness of these attitude angles is therefore obvious.
The most conventional methods for checking these attitude angles are based on measurement of the internal angles of a quadrilateral defined by measurement points, determined by means of sensing instruments applied and fixed to the individual wheels. The main drawbacks of these methods arise precisely from the fact of having to use sensing instruments fixed to the wheels, resulting, for example, in the problems or drawbacks listed below:
1. Mechanical assembly precision: possible errors during fixing of the instruments result in major measurement errors. Moreover, because of the precision required, whenever measurement must be performed, a significant portion of the testing time is used for this preliminary operation;
2. Stable fixing: the instrument for sensing, the measurement point must be fixed very firmly because, otherwise, it may be displaced during measurement and result in further errors;
3. Systematic errors during measurement: in view of the risk that the reference point on wheel may not be chosen in a sufficiently accurate manner, errors in measurement which cannot easily be controlled may arise;
4. Handling of the instruments: the instruments to be fitted must be light, so that they car be easily handled, but at the same time robust to take account of the environment in which they are used.
In order to overcome these problems, more modern methods for measuring the attitude angles have already been proposed, said methods being based on the use of opto-electronic measuring instruments which therefore do not make contact with the wheels of the vehicle or with the vehicle itself. Measuring instruments of this type have been proposed, for example, in U.S. Pat. No. 4,899,218, DE-A 2948573 and EP-A2-0,895,056.
U.S. Pat. No. 4,899,218 describes a measuring process based on the projection of a light beam with an oscillating structure onto the wheel so as to produce at least two reference images thereon; these images are read by video cameras positioned at a certain angle with respect to the optical plane defined by the plane of oscillation of the beam. The spatial position of the lines, and hence the wheel, is calculated by a processor using a known triangulation system.
DE-A-2948573 describes an apparatus in which the measuring process is based on a pair of images of the edge of the wheel rim, taken from different angles. From these images it is possible to obtain ellipses, the parameters of which may be used to determine the spatial position of the wheel, knowing the position and exact orientation of the video cameras.
EP-A2-0,895,056 describes a measuring apparatus and method consisting in obtaining a certain number of pairs of images of each wheel as the vehicle is moving towards the measurement position. The apparatus uses two video cameras for each wheel, having their lenses directed towards the measurement position and stably connected to the latter. The video cameras are connected to a processor able to process the images of each wheel viewed by them, so as to determine the spatial position of the rim by means of examination of the circular structure images and identification of the regions in which the transition between different grey levels is maximum. It is also possible to detect any wobble of the rim edge, i.e. non-perpendicularity of the plane of the wheel with respect to the associated axis of rotation, and use this to calculate the characteristic attitude angles.
All these known devices, however, are relatively complex as regards both the manufacture and initial assembly and therefore have a relatively high cost. Moreover, it does not appearxe2x80x94at least until nowxe2x80x94that they have found an effective practical use.
The object of the present invention is therefore to propose a system for measuring the characteristic attitude angles of the wheels of a vehicle, which is able to overcome these problems and make the measuring operation itself easy and rapid.